This invention relates to a method for producing a novel transparent paper particularly suitable as tracing paper or copying paper mainly used in drafting and reproduction and, further, as duplicating original to be made in combination with known reproduction processes such as diazotype, silver halide photography, and electrophotography.
Transparent papers conventionally used for the above purposes include:
(a) tracing papers made from highly wet-beaten natural cellulosic pulp, or said tracing papers further treated with, for example, a polymeric substance; PA1 (b) transparent papers made from relatively opaque papers other than tracing papers by impregnating with a fat or an oil, plasticizer, liquid paraffin, or the like, or by further surface treatment of said impregnated papers; PA1 (c) those made from a plastic film used as base material by surface modification; and PA1 (d) those manufactured by hot pressing a web formed from a mixture of thermoplastic synthetic resin pulp and natural cellulosic pulp, thereby to transparencize the web.
Transparent papers of the type (a), although used most widely, are inferior in dimensional stability and liable to curl, owing to their high sensitivity to humidity. Further, they have other defects in water resistance, tearing strength, etc. Members of the group (b) are superior to those of the group (a) in water resistance, tearing strength, and dimensional stability. However, when the paper is stored for a long period of time or is exposed to heat in the reproduction unit, the transparencizing agent tends to migrate, resulting in feathering of the image. Most of the papers of this type have further defects in writing quality and recovery from crease. The transparent papers of the group (c), although excellent in transparency, dimensional stability, and mechanical strengths, are very expensive and raises problems in waste disposal. Although improved in dimensional stability, the papers of the group (d) have disadvantages in that owing to uneven distribution of the synthetic pulp and the natural pulp, there are obtained papers having not uniform transparency but opaque speckles scattered about throughout the paper. Moreover, a special hot press installation is necessary in their manufacture.
As known well, transparency may be imparted generally by filling the pores of cellulosic paper with a substance having a refractive index approximating that of cellulosic fiber. However, if a liquid or low-melting solid substance is used as the transparencizing agent, a defective paper similar to those of the above-noted group (b) is obtained, while if a high-melting polymeric substance is used, increased viscosity of the transparencizing solution makes it difficult for the solution to penetrate into the cellulosic paper, thus leading to non-uniform transparency of the converted paper.
Accordingly, it is considered best at present to use as the transparencizing agent a reactive resin or a monomer which has a relatively low molecular weight so that it may penetrate sufficiently into the cellulosic paper and thereafter may be converted into a polymer by use of a catalyst, heat, and other means.
There is a precedent for the use of such a transparencizing agent. However, the polymers used were such thermosetting resins as phenol-formaldehyde resins, urea-formaldehyde resins, melamine-formaldehyde resins, and alkyd resins which had low rate of hardening. Although the rate of hardening can be increased to some extent by elevating the temperature or adding a catalyst, severe hardening conditions are undesirable because of their adverse effect of accelerating deterioration of the paper itself.